mai99dgf at studserv.uni-leipzig.de
Tue Oct 19 11:42:26 EDT 2004
On Tue, 19 Oct 2004 15:33:25 +0100, Simon Peyton-Jones <simonpj at microsoft.com> wrote:
> Thanks to those who responded to this thread about 'deriving'. My current thoughts are:
> * I'd be happy to add the ability to separate a 'deriving' clause
> from its data type declaration, if we can agree syntax
> (see below). It's fairly easy to do; it makes the language more
> orthogonal; it's useful.
> But in fact I think only Martin Sjögren has explicitly said that
> the feature would be useful.... and every feature has a cost.
It would be quite useful indeed!
The scenarios I have experienced are quite complex, but they occur.
> * I'm not at all keen on making '..deriving( Foo )' mean
> $(derive 'Foo) or something like that. Just make the TH
> call yourself!
> * No one is arguing hard for instance declarations in hi-boot
> files, so let's leave that for now.
> Re syntax, the obvious possibility (A) is to add
> derive( pred1, .., predn )
> as a new top-level declaration. E.g.
> derive( Typeable (T a) )
> But that means adding 'derive' as a keyword. Other possibilities:
> deriving( Typeable (T a) )
> -- (B) Re-use 'deriving' keyword
> The trouble with (B) is that the thing inside the parens is different in this situation than in a data type declaration.
> Any other ideas?
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